The US Government Federal Communications Commission's (FCC) more recent allocation of wireless radio frequency spectrum included moving or relocating various regional/national terrestrial broadcast services to lower frequency bands, in order to provide a structured opportunity for broadband multi-band wireless services in support of homeland security, land mobile radio for first responders, and fixed and mobile personal or commercial voice, video and data communications. The structural bandwidth allocated for these new services was arranged in a manner where hardware and system designers can utilize licensed carrier frequencies operating within a broadband contiguous spectrum in conjunction with other frequency bands separated, but related by a multiple or fractional order. Additionally, the upper portion spectrum was aligned with the standard for Universal Mobile Telecommunication Services (UMTS) in attempt to provide universal standardization across the globe. The acronym for this assembly of spectrum and for the intended application, is commonly referred to as 4G or LTE, or in some cases 4G/LTE. “4G” defined as the acronym for fourth generation cellular service, and LTE referring to Long Term Evolution, implying that the digital modulation protocol is intended to be a continuously evolving global standard for communications. Various radio systems incorporating digital voice, data and location services are evolving in combination with requirements to operate simultaneously across multiple bands in the VHF (100-200 MHz), UHF (380-520 MHz) and 700-900 MHz spectrum, in support of public safety and homeland security initiatives. These initiatives are spurring inventors toward multiple band antenna system designs to augment cooperative communications amongst multiple local, regional and national safety and security officials.
For example, cellular carriers were originally licensed to operate in the spectrum of 800 MHz (806-894 MHz) using traditional analog advanced mobile phone technology. With years of experience along with the introduction of enhanced digital modulation schemes, they can now provide advanced cooperative services in the 1700-1900 MHz bands. This was all made possible after several years and rounds of auctions, hosted by the FCC. These higher bands provide an approximate mathematical doubling (2×, and in some bands 3×) of the original 800 MHz bands. Furthermore, by extending these separate bands (800 and 1700-1900 MHz) into locally adjacent bands operating with advanced digital modulations, the spectral capacity is greatly increased. This of course is dependent upon hardware designers achieving efficient design platforms that meet the performance objectives established by the system architectural requirements. This hardware, operating with expanded frequency spectra, delivers voice and various data content via increased speed (bandwidth) and digitally encrypted capabilities to emergency personnel and end user public and private subscriber telecommunication services. Technological strides achieved in the consumer cellular markets combined with the fact that their respective spectra are interlaced with adjacent land mobile and public safety bands, increasingly build interest within the wireless industry to interlace or overlay cellular communications with the land mobile and public safety segments, regardless of the regulatory and technical challenges. The cellular communication services are currently operating in the 700-900 MHz and 1700-2200 MHz bands, whereas Private Land Mobile and Public Safety services operate in the 100-225 MHz, 380-520 MHz and 740-870 MHz bands.
Traditional monopole antennas are implemented in a variety of configurations for ground plane dependent wireless radio applications. Monopole radiators (e.g., monopole antennas) are often referred as “quarter-wave” antennas due to their characteristic requirement of their physical length approximating one-fourth (¼) wavelength at the desired frequency of operation, and are considered to be one of the most fundamental structures to achieve efficient omni-directional Radio Frequency (RF)/Microwave radiation. Monopoles also provide reasonably broad band performance relative to their desired operational frequency, and can be designed for efficient radiation in excess of 25% to 30% of total operational bandwidth.
Monopoles can be comprised of a conductive thin diameter wire radiator (primary conductor) oriented in a vertically normal position with respect to a close proximity conductive ground plane surface (secondary conductor). The ground plane is typically several wavelengths in diameter or infinitely sized for theoretical considerations. RF voltage is applied across the two conductors through a small isolated feed point near the center of the ground plane. It is important to note that the monopole antenna cannot physically exist without the ground plane. The ground plane is an integral part of the monopole impedance and radiation characteristics. Theoretically, the monopole is defined by its quarter-wave length size emanating from the existence of an infinite (very large) ground plane and defined by image theory of a virtual source on the opposite side of the ground plane, establishing dipole like characteristics.
Designing the monopole requires a design methodology to implement a vertical radiator approximating the desired one-fourth (¼) wavelength structure, and is a well known practice to those skilled in the art of antenna design. Furthermore, enhancing the bandwidth, radiation efficiency and reducing the physical height (length) of the monopole enable great flexibility in their employment.
A common design implementation includes top loading the monopole by physically increasing the diameter of the primary conductor at the highest point (maximum RF voltage) which effectively reduces the total physical height while simultaneously increasing the electrical length. The top load implementation results in a shorter physical radiator, operating at a lower and much broader RF frequency range. Other bandwidth enhancing techniques include increasing the physical diameter of the primary conductor, in effect decreasing the Length-to-Diameter (L/D) ratio with a benefit to reducing the total physical height and increasing operational bandwidth.
Mobile antennas and specifically, mobile monopole antennas are prominently utilized in various arenas. For example, mobile antennas are employed in the areas of Land Mobile Radio (LMR), public safety, homeland security, cellular, telematics, telemetry, in-building, portable applications, and the like. Such mobile antennas can be mounted using a physical mount to a surface or a magnet temporarily attached to a surface, etc. Yet, one mount technique has come to fruition as a standard for mobile antennas. In particular, a New Motorola™ (NMO) mount (herein referred to as the NMO mount) has become the industry standard for mobile antenna mounts, specifically mounting mobile antennas to automobiles.